The present invention relates to coated ink jet recording sheets and coating compositions used to prepare the same. In particular, the invention relates to coating compositions suitable for preparing glossy ink jet recording sheets which possess good printability characteristics.
Ink jet printing processes are well known. Such systems project ink droplets onto a recording sheet, e.g., paper, at varying densities and speed. When using multi-color ink jet systems, the process projects in very close proximity a number of different colored inks having varying properties and absorption rates. Indeed, these multi-color systems are designed to provide images which simulate photographic imaging, and such images require high resolution and color gamut. Accordingly, ink jet recording sheets must be able to absorb ink at high densities, in a capacity such that the colors deposited are bright and clear, at rates to effect quick drying, absorb ink so that it does not run or blot, and in a manner that results in smooth images.
To meet these goals, highly porous pigments, e.g., porous silicas, have been incorporated into paper coatings. Such silica-based coating systems have been successful in meeting printability goals. However, it has been difficult to obtain such properties and produce a non-matted, or glossy, finish typically seen in traditional photographic systems. The aforementioned porous pigments typically have porosities above 1 cc/g and have average particle sizes greater than 1 micron. Such particle sizes and porosities increase the surface roughness of the finished coating, thereby deflecting incident light so that it is scattered, thereby matting the coating.
To enhance the glossiness of such coatings, second gloss layers are provided on top of ink receptive layers prepared from the aforementioned porous pigments. These top layers are prepared from binder systems that are inherently glossy, or from layers comprising binder and much smaller sized inorganic oxide particles, e.g., conventional colloidal silica. In the latter approach, the colloidal silica tends to enhance the ink receptive nature of the top coating, but does not have large enough particle size to cause significant surface deformation. There is, however, a tendency for these colloidal particles to agglomerate at high concentrations, thereby causing imperfections and surface roughness in the top layer, and thereby reducing gloss. Accordingly, lower silica concentrations (i.e., lower colloidal solids to binder ratios) have been used when colloidal silica is employed in a top glossy layer.
It has recently been discovered that colloidal silica having relatively low amounts of alkali metal ions, e.g., sodium, does not aggregate in relatively high solids content coating formulations. Deionized colloidal silica is such an example. By “deionized,” it is typically meant that any ions, e.g., metal alkali ions such as sodium, have been removed from the colloidal silica solution to an extent such that less than 1000 ppm alkali ions as measured by inductively coupled plasma (ICP) techniques is present in the colloidal silica. Such colloidal silicas are commercially available from W. R. Grace & Co.-Conn. as Ludox® TMA having a pH of 5.0 at 25° C. Coatings prepared from such colloidal silicas are glossy and have printability properties which are acceptable in particular applications. However, they do not have excellent printability properties sought in other segments of the ink jet market.
It would therefore be quite desirable to increase the amounts of solid inorganic oxides in these top layers to further improve printability. Indeed, it would be desirable to use coating layers having at least 1:1 pigment to binder solids ratios, and even more preferable to employ coatings having pigment to binder ratios as high as 4:1 to achieve excellent printability, yet at the same time attain acceptable gloss.